Process for the production of high apparent density water atomized steel powders

ABSTRACT

The apparent density of molding-grade, water atomized steel powder can be significantly increased by employing the following prescribed treatment. Coarse particles are removed in order that at least 80% of the initial powders are finer than 80 mesh. The size distribution of the powders is then determined. The powders are then annealed to both reduce the carbon and oxygen contents and soften the particles. The annealed and agglomerated particles are then ground in a disk mill at specified speeds and gap spacings, depending on the size distribution of the initial powders. Apparent densities in excess of 3.2 may be achieved by (a) employing powders with a coarser particle size distribution, (b) increasing the rotational speed of the disks and (c) decreasing the mill gap.

Elite States atent 1 1 Chao et al. 1 Aug. 19, 1975 [541 PROCESS FOR THE PRODUCTION OF 3,725,142 4/1973 Huseby 148/126 H APPARENT DENSITY WATER 3.764.295 10/1973 Lindskog et a1. 75/0.5 C

ATOMIZED STEEL POWDERS [73] Assignee:

[22] Filed:

Inventors: Hung-Chi Chao; John H. Gross,

both of Monroeville; Robert R. Judd; Roger L. Rueckl, both of Murrysville, all of Pa.

United States Steel Corporation, Pittsburgh, Pa.

Aug. 16, 1973 Appl. No.: 389,603

Primary ExaminerW. Stallard Attorney, Agent, or Firm-Arthur J. Greif [57] ABSTRACT The apparent density of molding-grade, water atomized steel powder can be significantly increased by employing the following prescribed treatment. Coarse particles are removed in order that at least 80% of the initial powders are finer than 80 mesh. The size distribution of the powders is then determined. The powders are then annealed to both reduce the carbon and oxygen contents and soften the particles. The annealed and agglomerated particles are then ground in a disk mill at specified speeds and gap spacings, depending on the size distribution of the initial powders. Apparent densities in excess of 3.2 may be achieved by (a) employing powders with a coarser particle size distribution, (1)) increasing the rotational speed of the disks and (c) decreasing the mill gap.

6*Claims, 2 Drawing Figures POWDER SIZE CHARACTER/ST/C ///6 INCH MILL GAP l 1 l l 1 M/LL SPEED R. P. M.

///6 INCH M/LL GAP MILL SPEED R. P. M.

PATENT AUG 1 9 I975 SIZET 2 0? 2 w w I P A 04 0 G 53 0 I L 3 5 L 3 I M H. w 6 O N 3 4 m 2. w I 53 5 0M 2 3 53 0 0 -0 3 2 0 l0 5 0 w 00 000 000 0 00 00 jj w ww jfi zm wjmfijm 44443331531333 2222222 MILL SPEED RRM.

PROCESS FOR THE PRODUCTION OF HIGH APPARENT DENSITY WATER ATOMIZED STEEL POWDERS This invention is directed to an economical method for the production of water atomized steel powders with a high apparent density, and more particularly to a method for increasing the apparent density of such water atomized particles. 1 I,

Various methods are now employed for the production of metal powders. Thus. metal powders may be produced by (a) electrolytic deposition. (b) direct reduction of metal oxides. (c) reduction of metal halides. and by (d) atomization with high pressure fluids. e.; water and inert gases. For the production of molding grade-steel powders in large quantities. metal oxide reduction and water atomization are considerably more economical. Of the latter two methods, steel powders that are water atomized have a generally lower metalloid contentsWater atomized powders also exhibit better flow rates. i.e. better pressfeeding efficiency and therefore permit higher production rates'in the production of'powder-metallurgy partsj U.S. Pat. No. 3.325.277 exemplifies such a water-atomization process. Although such a process offers a number of commercial advantages. it is somewhat limited in the range of mechanical properties of powders which can be produced thereby. Thus. the apparent densities of available water atomized steel powders is generally within the range of 2.8 to 3.2 gms/cc. Apparent density is determined by measuring the weight of powder in a calibrated cup. Since the density can be effected by the mode of packing, this measurement has generally been standardized (ASTM 13212-48), i.e. by flowing the powder through a 0.1 inch diameter by 0.125 inch long orifice located one inch above thetop surface of a 25 cc cup.

A method for the production of high apparent density cast iron shot is disclosed in U.S. Pat. No. 3,597,188. However, this method is limited to the use of coarse powders which are brittle, by reason of their extremely high carbon contents. Since the bulk of powders produced by water atomization are finer than 80 mesh (with a major portion finer than 200 mesh), this latter process is uneconomical because it'requires the discarding of more than half of the initially produced powders. Equally important, the' disclosed process is not appropriate for the production of steel powders, i.e. those with carbon contents below about 1.7%.

It is therefore a principal object of this invention to provide a method for producing water-atomized steel powders with an apparent density greater than 3.2 gm/cc. and preferably greater than 3.4 gm/cc.

1 It is a further object to. provide a method for producing'water-atomized steel powders, with an apparent density equal to or greater than 3.2 gm/cc, wherein a major portion of the as-atomized powders can be utilized therein.

It is another object of this invention to. provide a method for the production of molding grade steel powders which exhibit a combination of high apparent den sity and a green strength sufficient to pemiit normal handling before sintcring.

These and other objects and advantages of the instant process will be more readily understood from the following detailed description, when read in conjunction with the appended claims and drawings, in which:

FIG. 1 is a graphical representation showing the effect of particle size distribution, disk'speed and a millgap of l/l6-inch 0n apparent density.

FIG. 2 is a graphical representation showing the effect of particle size distribution, disk speed and a millgap of l/64-inch on apparent density.

The method of this invention is applicable to wateratomized steel powders from virtually any source. Water-atomized steel powders generally contain impurities, primarily in the form of oxides, that must be removed before the powder has commercial value for the production of powder-metallurgical parts. In order to produce steel powders with maximum compressibility, it is also desirable that the final powders have a carbon content below about 0.10%, and preferably below 0.01%. However, it is generally impractical to provide an initial steel melt with such a low carbon content. Therefore, such steel melts may contain up to 0.8% C, but preferably less than about 0.15% C, and the carbon content of the atomized powders is thereafter lowered by annealing in a decarburizing atmosphere. Atomization by high pressure water jets results in rapid quenching of the liquid metal droplets during the early stage of the atomization process. Therefore, even if a relatively low carbon steel were employed (i.e. eliminating the need for decarburization) in the atomization process, it would still be necessary to anneal the powders to effect both softening and lowering the oxygen content thereof (to a value below about 0.2%). The initial oxygen contents of the as-water-atomized particles is generally far in excess of 0.2%, generally about 1.0%. As a result of this high surface oxygen content and the particle configuration thereof, the as-atomized particles will pack to a high apparent density, i.e. well in excess of 3.2 gm/cc. However, after the requisite annealing and reduction of the oxygen content;thereby, the apparent density will be within the range of 2.8 to 3.2 gm/cc. Annealing is conducted at temperatures of 1400F to 2100F, in a reducing atmosphere such as hydrogen or dissociated ammonia for a time sufficient to effect the desired softening and reduction of impurities. This annealing treatment not only purifies the steel powder, but causes the particles to stick together in the form of a sintered cake, thereby necessitating a breaking up of the cake to return it to powder consistency. 1n the process of U.S. Pat. No. 3,325,277, this requisite break-up is'aecomplished in a hammermill; employing impact shattering to return the particles to their original as-atomized size. The instant invention departs from this process by performing a true grinding operation in a disk mill; employing a shear mechanism for comminution. It has now been found that by regulation of such a grinding operation, the final apparent density can be tailored to specific requirements, depending on the size distribution of the original as-atomized particles. Y I' l v I The size distribution of the as-atomized powder may be determined by conventional screen analysis. This screen analysis is then employed to develop the particle size characteristic (PSC) of the powder. It has been found that unduly coarse, as-atomized particles cannot be ground to achieve the desired objects of this invention. Thus, to achieve the requisite grinding, it is necessary that at least and preferably greater than of the as-atomized particles be finer than 80 mesh (U.S.

Series). While a number of different methods are available for defining PSC value, for purposes of this invention, this value is determined in the following manner.

U.S. Standard Cumulative Mesh Retained Retained 230 4.9 28.7 325 12.9 41.6 Pan 58.4 100.0

Therefore, the PSC of this powder would be 204.2/100, or 2.04. Water atomized particles with PSC values of about 2.0 to 4.0 may be effectively employed in the instant process. Once the PSC is known, and the powder has been annealed, a grinding cycle can be established to tailor the properties to specific requirements. A Disk Attrition Mill is then employed to effect the requiste grinding. As a result of annealing, the particles sinter together in the form of a cake. If necessary, the resultant sinter cake is first broken in pieces small enough, generally less than about one inch, to be fed into the Disk Attrition Mill. In such a mill, grinding occurs between disks, which generally rotate in either a vertical or horizontal plane. The feed enters near the center of the disk, travels by centrifugal force to the peripheral, grinding plate portion thereof, and is then discharged. While in certain disk mills, spike tooth plates have been employed, it should be understood that such plates are not applicable to the instant invention, which is limited to the use of conventional, friction grinding plates. The mill gap referred to herein, is the distance between the grinding plates. The disk mill is particularly suited for the purposes of this invention since it has been found that such a mill is capable of yielding a controlled and predictable degree of grinding which is basically a function of (a) the mill gap, and (b) the linear speed of a point on medial radius r of the grinding plates. In a disk mill, the locus of the grinding plates form a ring, (i.e. two concentric circles); where the distance from the center to the grinding plate, i.e. from the center to inner circle is r The distance from the center to the peripheral portion of the grinding plate, i.e. from the center to the outer circle, is r Therefore, the medial radius r,,, is then r, +r2/2. Since linear speed, v. is equal to the angular speed (w) times the radius, the linear speed of a point on the medial radius may easily be determined from the revolutions per minute of the grinding plates. Thus, for example, if grinding plates with a r of 12 inches are rotated at 3000 rpms, the linear speed (v) will be:

v 3000 X 2w X 12 72,0001? inches/min.

Through the use of statistical regression and engineering interpretation analysis, the effect of the above variables on the apparent density of the final product powder was found to be described by the equation:

Apparent Density (g/cc) 2.16 0.30 PSC 1.28

PSC is the particle size characteristic of the as-water atomized powder, prior to annealing v is the linear speed of a point on the medial radius of the grinding plates, in inches per minute, and

LG is the Log of (mill gap in inches).

Through the use of the above equation, a grinding cycle can therefore be established to tailor the properties of the final product powder to specific requirements. To provide a better understanding of the use thereof, the process equation was employed to develop the graphs of FIGS. 1 and 2 for a laboratory sized disk mill with a 13-inch diameter disk, having a r,,, of 5.31 inches. For ease of interpretation, (i.e. the avoidance of highly cumbersome numbers) the linear speed v, was converted to the rpm of this disk mill. It should be understood, however, that these graphs are only applicable 'to a mill with a r of 5.31 inches. In commercial practice, a larger diameter disk mill would generally be employed. The curves of FIGS. 1 and 2 would then be shifted to lower rpm values as the size of the mill is increased, since the linear v (at any given rpm) would be correspondingly higher. In general, such mills will be operated at speeds of about 200-5000 rpm, with mill gaps ranging from about 0.01 to 0.10 inches.

The utilization of the graphs of FIGS. 1 and 2 will be described for powders exhibiting the following exemplary screen analyses: I

Powder i +200 +230 +325 If as-atomized powder A were to be employed, and the mill were to be operated at a gap of l/ 16 inch (FIG. 1 it may be seen, for example, that apparent densities of 3.2 gm/cc and 3.45 gm/cc could be achieved by employing speeds of 2500 rpm and 3650 rpm respectively.

The effect of reduced mill gap may be seen by comparison with FIG. 2 If the same powder (A) were to be employed, similar densities could be achieved at materially lower disk speeds. Thus, a speed of only 1600 rpm would produce a product with a density of 3.2 gm/cc, while a density of 3.45 gm/cc would be achieved with disks operated at a speed of 2775 rpm. Powder B, being inherently finer, cannot be as readily used (in a small diameter mill) to produce high apparent densities. Nevertheless, if the mill gap is reduced to l/64 inch, as in FIG. 2 the equivalent apparent densities can be produced at disk speeds of 3225 rpm and 4050 rpm respectively.

From the illustrative examples above (or from the process equation itself) it may therefore be seen that apparent density increases as:

a. the PSC value of the as-atomized particles is increased,

b. the disk speed of the mill is increased, and

c. the mill gap is decreased.

It was also found, within the specified temperature range of l4002l00F, that apparent density could be slightly increased by decreasing the annealing temperature. However, the use of lower temperatures would necessitate the employmemt of longer annealing periods. As a practical compromise of these competing effects, i.e., the achievement of high apparent density within a reasonably short annealing time, it is therefore preferable to anneal within the range of about l700 to 1900F.

We claim:

1. A method for the production of molding grade steel powders with high apparent densities, which comprises,

a. providing as-water-atomized steel particles with a prescribed size distribution, wherein at least 80% of said particles are finer than 80 mesh and said distribution exhibits a PSC value of between 2.0 to 4.0,

b. annealing said particles at a temperature within the range of l4002l00F for a time at least sufficient to (i) effect the desired softening thereof, and (ii) reduce the oxygen content thereof to a value below about 0.2 weight percent, said annealing causing said particles to sinter together,

c. feeding the annealed, sintered particles to a disk mill operated at a speed of between about 200 to 5000 revolutions per minute and a mill gap of between about 0.01 to 0.10 inches, wherein the linear speed v of said disks is sufficiently high and the mill gap G is sufficiently close to grind said sintered particles to molding grade powders with an apparent density in excess of 3.2 gm/cc, substantially all of which are finer than mesh.

2. The method of claim 1, wherein said linear speed v and said mill gap G are correlated with the PSC value of said particles in accord with the following equation:

3. The method of claim 2, wherein at least of said as-water-atomized particles are finer than 80 mesh, with a major portion finer than 200 mesh.

4. The method of claim 2, wherein the carbon content of said as-atomized particles is less than 0.15% and said annealing temperature is between about l700 and 1900F.

5. The method of claim 4, wherein the PSC value of said atomized particles is greater than about 2.5 and the speed v and mill gap G are correlated with said PSC value in accord with the following equation:

to yield a molding grade powder product with an apparent density greater than 3.4 gm/cc.

6. The method of claim 5, wherein at least 95% of said as-water-atomized particles are finer than 80 mesh, with a major portion finer than 200 mesh. 

1. A METHOD FOR THE PRODUCTION OF OLDING GRADE STEEL POWDERS WITH HIGH APPARENT DENSITIES, WHICH COMPRISES, A. PROVIDING AS-WATER-ATOMIZED STEEL PARTICLES WITH A PRESCRIBED SIZE DISTRIBUTION, WHEREIN AT LEAST 80% OF SAID PARTICLES ARE FINER THAN 80 MESH AND SAID DISTRIBUTION EXHIBITS A PSC VALUE OF BETWEEN 2.0 TO 4.0, B. ANNEALING SAID PARTICLES AT A TEMPERATURE WITHIN THE RANGE OF 1400*-2100*F FOR A TIME AT LEAST SUFICIENT TO (I) EFFECT THE DESIRED SOFTENING THEREOF, AND (22) REDUCE THE OXYGEN CONTENT THEREOF TO A VALUE BELOW ABOUT 0.2 WEIGHT PER CENT, SAID ANNEALING CAUSING SAID PARTICLES TO SINTER TOGETHER, C. FEEDING THE ANNEALED, SINTERED PARTICLES TO A DISK MILL OPERATED AT A SPEED OF BETWEEN ABOUT 200 TO 5000 REVOLUTIONS PER MINUTE AND A MLL GAP OF BETWEEN ABOUT 0.01 TO 0.10 INCHES, WHEREIN THE LINEAR SPEED V OF SAID DISKS IS SUFFICIENTLY HIGH AND THE MILL GAP G IS SUFFICIENTLY CLOSE TO GRIND SAID SINTERED PARTICLES TO MOLDING GRADE POWDERS WITH AN APPARENT DENSITY IN EXCESS OF 3,2 GM/CC, SUBSTANTIALLY ALL OF WHICH ARE FINER THAN 80 MESH
 2. The method of claim 1, wherein said linear speed v and said mill gap G are correlated with the PSC value of said particles in accord with the following equation: +0.30 PSC -1.28 .10 5v +2.87 .10 2 LG +1.93 .10 6 .v PSC +4.00 .10 11 .v2 -3.96 .10 6 .v. LG > 1.04
 3. The method of claim 2, wherein at least 95% of said as-water-atomized particles are finer than 80 mesh, with a major portion finer than 200 mesh.
 4. The method of claim 2, wherein the carbon content of said as-atomized particles is less than 0.15% and said annealing temperature is between about 1700* and 1900*F.
 5. The method of claim 4, wherein the PSC value of said atomized particles is greater than about 2.5 and the speed v and mill gap G are correlated with said PSC value in accord with the following equation: +0.30 PSC -1.28.10 5v +2.87.10 2 LG +1.93 .10 6 .v.PSC +4.00 .10 11 v2 -3.96 .10 6 .v. LG > 1.24 to yield a molding grade powder product with an apparent density greater than 3.4 gm/cc.
 6. The method of claim 5, wherein at least 95% of said as-water-atomized particles are finer than 80 mesh, with a major portion finer than 200 mesh. 